A vapor retarder is defined by ASTM Standard C 755 as a material or system that adequately retards the transmission of water vapor under specified conditions. The permeance of an adequate retarder for residential construction will not exceed 1 perm. A perm rating is a measure of the diffusion of water vapor through a material. Vapor diffusion accounts for only a small amount of the total moisture in a building. Therefore, other means should be utilized to reduce water vapor migration due to air infiltration.

An air retarder is different from a vapor retarder in that it blocks only air and liquid water,not water vapor. House wraps are one form of an air retarder. Typical exterior house wraps are not vapor retarders.